Light emitting diodes or “LEDs” are semiconductor devices which are used as light emitting elements in indicators, displays and instruments. Increasingly, LED's are being used as replacements for ordinary light bulbs as, for example, in automobile brake lights and traffic lights. LED's provide numerous advantages including compactness and efficient conversion of electrical energy to light.
Conventional LED's include thin layers of semiconductor material of two opposite conductivity types disposed in a stack, one above the other, with one or more layers of n-type material in one part of the stack and one or more layers of p-type material at another part of the stack. The LED includes a junction at the interface of the p-type and n-type layers. The various layers of the stack may be deposited in sequence on a substrate, such as a sapphire substrate, to form a wafer. The wafer is then cut apart to form individual dies which constitute separate LEDs. The substrate constitutes the bottom or rear of the stack. The n-type and p-type layers typically are quite thin, as, for example, a few tenths of a micron to a few microns thick, whereas the substrate typically is thick enough to survive handling during manufacture as, for example, a few tenths of a millimeter thick. Appropriate electrodes are provided for applying an electrical current through the p-type and n-type layers of the stack, so that the current passes through the junction. Each such electrode typically includes a pad suitable for attachment of wires or leads. Typically, one electrode is disposed on the top or front surface of the stack, remote from the substrate, whereas another electrode is disposed on the substrate or on a layer close to the substrate, so that the electrodes are disposed on opposite sides of the junction. The electrode disposed on the top of the stack may include a transparent conductive layer to enhance distribution of the current over the horizontal extent of the stack.
The stack structure typically is mounted in a package which supports and protects the structure during use. For example, the rear surface of the substrate may be mounted to a metal reflector by an epoxy or other adhesive, commonly referred to as a “die bond”. A transparent encapsulant may be provided over the stack and reflector.
In operation, electric current passing between the electrodes and through the LED is carried principally by electrons in the n-type layer and by electron vacancies or “holes” in the p-type layer. The electrons and holes move in opposite directions toward the junction, and recombine with one another at the junction. Energy released by electron-hole recombination is emitted from the LED as light. As used in the present disclosure, the term “light” includes visible light rays, as well as light rays in the infrared and ultraviolet wavelength ranges. The wavelength of the emitted light depends on many factors, including the composition of the semiconductor materials and the structure of the junction.
The term “external quantum efficiency” as used with reference to an LED refers to the ratio between the amount of energy emitted to the outside of the stack or to the outside of the package and the amount of energy supplied by the electrical current passing through the device. Of course, it is desirable to maximize the external quantum efficiency of an LED. Not all of the light which is emitted at the junction reaches the outside of the stack or the outside of the package. The term “extraction efficiency” refers to the ratio between the amount of light emitted at the junction and the amount of light which reaches the outside of the stack or package. All else being equal, increasing the extraction efficiency directly increases the external quantum efficiency.
The light emitted at the junction normally is emitted in all directions from each point within the junction. Light passing vertically upwardly from the junction escapes from the stack through the top of the stack. However, the bonding pad of the top electrode typically is opaque, and blocks some of the light which passes upwardly, toward the top of the stack. Moreover, where a transparent electrode is employed, the transparent electrode does not have perfect transparency, and hence absorbs some of the light passing upwardly. Light passing upwardly from the junction at a shallow oblique angle, close to the horizontal direction, can be reflected at the top surface and redirected downwardly into the stack and toward the edges of the stack. Some of the light passes out of the stack at the edges. Also, about one-half of the light emitted at the junction is initially directed downwardly toward the substrate. Where the substrate is opaque, this light is absorbed in the substrate. Where the substrate is transparent, but the bottom surface of the substrate is mounted using an opaque die bond, this light is absorbed in the die bond. This last effect promotes degradation of the die bond during service, as well as reducing the external quantum efficiency of the device.
Various proposals to attack this problem have been advanced heretofore. Some of these proposals would require unconventional structures for mounting the stack structure in the package, which in turn would increase the cost. As disclosed, for example, in U.S. Pat. No. 5,939,735, it has been proposed to provide an LED having a transparent substrate with a reflective coating on the bottom or rear surface of the substrate. The reflective coating redirects light reaching the bottom of the stack upwardly through the stack. While such a reflective coating can increase the external quantum efficiency of the device somewhat, its effect is limited by the blocking effect of the top electrode pad and by absorption in the transparent layer at the top of the stack. Moreover, light reflected upwardly from the bottom of the substrate must pass through the junction. The junction typically has very high absorptivity for light at the emission wavelength. Thus, a considerable part of the light reflected from the bottom of the stack is absorbed and converted to useless heat in the junction, before it ever reaches the top of the stack.
Accordingly, despite all of the efforts which have been devoted in the art heretofore to increasing the extraction efficiency of LED's, still further improvements would be desirable.